Episode 8: Galaxies and black holes grew up together

If our previous episode left you wondering whether black holes are too strange to actually exist in the real Universe, you’ll be happy (or shocked) to find out that, by now, astronomers have quite convincing evidence that these exotic objects aren’t just a figment of someone’s fancy math skills. Because no light can escape from inside a black hole, most of the information used to conclude that black holes exist comes from the stuff that is on the verge of falling in, and is already experiencing the effects of extremely strong gravity near the black hole’s edge. Lonely, isolated black holes are therefore very hard to find. Most of the black holes that we know of right now were discovered because they are eating up material from their surroundings and, in the process of falling in, this gas gets heated up so much that it emits X-rays we can detect.

Stellar-mass black holes, which are the end-state of very very massive stars after they have run out of fuel (as described in Episode 1), typically feast on gas from a nearby companion star. Although our Sun is “single”, many stars in our Galaxy live in couples or triples (or even groups of up to seven!), so if one of the stars dies and leaves a black hole behind, this black hole can eat up gas from the outer layers of the remaining normal star(s) in the “family”. I know, it sounds a bit like stellar cannibalism, doesn’t it?

In 1964, astronomers found X-ray emission from an object in the constellation Cygnus, which appears to be the first discovered example of a black hole swallowing gas from a nearby star. Or, at least, we know that this object, called Cygnus X-1, consists of a blue supergiant star rotating around a very massive, very compact, but unseen companion. Gas from the visible star gets fed to the companion and heated up to produce one of the strongest X-ray sources seen from Earth. To the best of our understanding, this invisible companion is about 15 times more massive than our Sun and, for such a mass, it is too faint and too compact to be anything we know of other than a black hole (or something even stranger and less likely than that). Since the discovery of Cygnus X-1, the list of known similar objects, all probably hosting black holes feeding on the outer layers of their nearby stars, has gotten quite long already and continues to grow.

The gas probably does not fall straight down into the black hole but, much like water swirling down the drain, it spirals in forming a disk before it gets eventually assimilated. The gas is composed mostly of electrons and protons, but also contains traces of ions from other chemical elements, such as iron. Each ion prefers to emit at its own specific X-ray energies, which produces a set of “bumps” at these energies in the X-ray spectrum of the gas. That is to say, at some energies, where ions like to emit, the emission is brighter than at other energies. As the ions orbit close to the black hole, the general theory of relativity predicts a very specific, crooked shape of these bumps. Although there is still a little debate on the matter, it is generally accepted that these shapes have been in fact seen in X-ray spectra, providing an excellent test of Einstein’s equations, and proving that what we are seeing in X-rays is indeed gas that is really close to the edge of a black hole.

Nobody really understands exactly how this happens, but it seems that not all the matter in the disk gets swallowed by the black hole: some of it is accelerated (probably by a twisted geometry of the magnetic fields) in really energetic jets perpendicular to the surface of the disk. Below is an artist’s impression of what Cygnus X-1 might look like in all its glory, disk and jets included.

Artist’s impression of a binary system akin to Cygnus X-1. It consists of a blue supergiant star (right) and a black hole. The black hole is surrounded by a gaseous accretion disk that is fed by the star. Some black holes emit jets along the polar axis, as shown here. Credit: NASA / Honeywell Max-Q Digital Group / Dana Berry

You’d be hard-pressed to see the jets of the black hole in Cygnus X-1 — they don’t look terribly exciting, even when looked at through the best telescopes on Earth. But there actually is a spectacular black hole jet that you can see, given a large amateur telescope and excellent observing conditions. This 5,000 light years long jet has been discovered almost one hundred years ago in the center of the galaxy M87 in the constellation Virgo, and it is created by a black hole that is unbelievably large — several billion times more massive than our Sun!

In fact, black holes seem to come in two “flavours”: ones which are only a couple of (tens of) times the mass of the Sun (“stellar-mass black holes”), and ones which are millions to billions of times bigger (“super-massive black holes”). Every large galaxy has a supermassive black hole at its centre — and our own Milky Way is not an exception. For the last almost twenty years, two different groups of researchers at the Max Planck Institute for Extraterrestrial Physics and at the University of California in Los Angeles have been using some of the largest telescopes on Earth, in Chile and on top of the extinct Mauna Kea volcano in Hawaii, to track the paths of the stars in the centre of the Milky Way with fantastic precision. Because the stars in that region of the sky are really close together, it took very sharp vision to tell individual stars apart — so sharp that, if (s)he had such a vision, an astronaut on the International Space Station could distinguish something the size of your iPhone on the ground. With these observations, both groups of researchers concluded that the stars in the centre of our galaxy are orbiting around something very very massive (about four million times the mass of the Sun), yet dark, and very compact (otherwise some of the stars would collide with it). According to our current understanding of physics, this object can be nothing else but a supermassive black hole.

Animation of the stellar orbits in the central 0.5 arcsec of the Milky Way. Images taken from the years 1995 through 2013 are used to track specific stars orbiting the proposed black hole at the center of the Galaxy. These orbits, and a simple application of Kepler’s Laws, provide the best evidence yet for a supermassive black hole, which has a mass of 4 million times the mass of the Sun.

If you find black holes fascinating, then supermassive black holes must be doubly so. First of all, it’s not yet entirely clear how these black holes grow to such huge masses. Sure, there is lots of gas to eat at the centre of large galaxies, and sometimes galaxies collide and their central black holes might merge into a single, bigger black hole. Even so, a stellar-mass black hole left over from the death of a star would need to be on a pretty copious diet for its entire life in order to transform itself into a supermassive black hole that is a billion times bigger. And, even supposing the “force-feeding” is successful, it’s not really clear why we don’t see any black holes with one thousand or one hundred thousand times the mass of the Sun. Possibly, it’s just because such “intermediate-mass” black holes are much harder to spot.

Another enigmatic part of the story is that the mass of the black hole at the centre of any given galaxy appears to be closely linked to the mass of that galaxy itself. It seems that there is an interesting interplay between the two: not only does the host galaxy affect the central black hole, but the black hole also has a say in the growth of its host galaxy. Galaxies and black holes truly grow up together! But how can a black hole with a size (event horizon) smaller than the Solar System influence the growth of an entire galaxy? This is still a matter of active research that we are both involved in, and you’ll be the first to know if we find the definitive answer. For now, the powerful jets blown out by the black holes (like the jet in M87) seem to be our best clue.

These jets blow huge bubbles into the surrounding gas (kind of, but only kind of, like blowing air into water through a straw) – as the gas is disturbed, it becomes less likely to form stars, so the host galaxy grows slower. This happens in cycles: as the black hole blows out the gas, it becomes starved for fuel, and the jets get weaker until the gas settles down again; then the black hole re-starts getting fed and produces the next pair of powerful jets, and so on. We believe that these cycles are the way in which a supermassive black hole can control the way its host galaxy evolves. As one of our renowned collaborators puts it, it’s like having something the size of a grape dictating the fate of something the size of the Earth.

Below is a famous example of bubbles blown by the central black hole in the galaxy NGC1275 in the constellation Perseus. You are looking at a detailed image of the X-ray emitting gas that would otherwise cool into stars, or get eaten by the black hole, but instead it has been disturbed several times and displaced from the centre of the host galaxy (the central bright spot is where the black hole is). The largest black hole inflated bubbles discovered so far measure an incredible 640,000 light years in diameter (they were found in a distant system in the constellation Camelopardalis), showing that the outbursts of black holes can be extremely powerful.

The central supermassive black hole in the galaxy NGC 1275 blows bubbles into the X-ray emitting gas seen in this image. In this way, the black hole can influence the growth of its host galaxy by displacing gas that would otherwise turn into stars. Credit: NASA/CXC/IoA/A.Fabian et al.

Even if we might not yet understand the exact details of how supermassive black holes manage to spread their influence over hundreds of thousands of light years, one thing has become very clear in our recent research: black holes are more than just exotic objects that suck stuff in, swallowing everything that gets in their way. They are the beating hearts of galaxies, playing a big role in their lives and development. Without supermassive black holes, the whole Universe as we know it would be a very different place!